Hydraulic anti-lock brake control system

ABSTRACT

A brake control system includes a pressure operated wheel brake, a master cylinder, a positive displacement pump drivingly connected to the wheel, a brake actuator providing a brake pressure to the wheel brake in proportion to the pump generated pressure, an orifice exhausting the pump generated pressure at a rate which prevents the pump generated pressure from operating the actuator throughout the range of vehicle speeds, and a slip control valve in series connection with the orifice and providing a variable restriction to exhaustion of the pump generated pressure in proportion to the difference between the master cylinder and wheel brake pressures. The flow area presented by the variable restriction of the slip control valve decreases at a decreasing rate as the differential between master and wheel brake pressures increases.

llnited States l-larned et a1.

[ HYDRAULIC ANTI-LOCK BRAKE CONTROL SYSTEM [75] Inventors: John L.Harned, Grosse Pointe Woods; Laird E. Johnston,

Birmingham, both of Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: Sept. 30, 1971 21] Appl. No.: 185,233

52 us. (:1. 303/10, 188/359, 303/21 F [51] Int. Cl B60t 13/16 [58] Fieldof Search 303/10, 11, 21F; 60/54.5 P;

[56] References Cited UNITED STATES PATENTS 3,661,427 5/1972 Hodge303/10 3,623,778 11/1971 Horvath 303/21 F 3,288,538 11/1966 DeMarcellus303/21 F Zfl 32 a l my Jan. 22, 1974 Primary Examiner-George E. A.Halvosa Attorney, Agent, orFirrn- D. D. McGraw [5 7 ABSTRACT A brakecontrol system includes a pressure operated wheel brake, a mastercylinder, a positive displacement pump drivingly connected to the wheel,a brake actuator providing a brake pressure to the wheel brake inproportion to the pump generated pressure, an orifice exhausting thepump generated pressure at a rate which prevents the pump generatedpressure from operating the actuator throughout the range of vehiclespeeds, and a slip control valve in series connection with the orificeand providing a variable restriction to exhaustion of the pump generatedpressure in proportion to the difference between the master cylinder andwheel brake pressures. The flow area presented by the variablerestriction of the slip control valve decreases at a decreasing rate asthe differential between master and wheel brake pressures increases.

1 Claim, 1 Drawing Figure PATENTED M22574 3,787, 094

Kw M

ATTORNEY HYDRAULKC ANTI-LOCK BRAKE CONTROL SYSTEM The invention relatesto an anti-lock brake control system and more particularly to an allhydraulic wheel slip control system which permits the vehicle operatorto control the level of wheel slip overa wide range of values withoutallowing complete wheel lock-up.

It is well known that the lateral friction coefficient between a vehicletire and the road surface decreases rapidly when a braking torquegenerated by an excessive brake pedal actuation causes the vehicle wheelto depart from the free rolling condition and approach the locked wheelcondition. This decrease in lateral friction coefficient as excessivebraking causes an increase in wheel slip is commonly evidenced byimpaired vehicle directional stability. It is, therefore, desirable tolimit the decrease in lateral friction coefficient by controlling thebrake pressure so as to maintain the wheel slip at some acceptablevalue.

The present invention allows the vehicle operator to control themagnitude of wheel slip over a wide range of values but prevents thewheels from looking up completely.

The present invention includes generally a fluid pressure operated wheelbrake, an operator actuated master cylinder, a positive displacementpump driven by the wheel, an actuator providing abrake pressureproportional to the pump generated pressure, an orifice exhausting thepump generated pressure to the pump at a rate which preventsthe actuatorfrom pressurizing the brake throughout the range of pump speedsconsistent with vehicle operation, and a slip control valve in seriesconnection with the orifice and providing a vari-' able restriction as afunction of the difference between the master cylinder and wheel brakepressures. When operator actuation of the master cylinder results in amaster cylinder pressure exceeding the wheel brake pressure, the slipcontrol valve decreases the flow area exhausting the pump generatedpressure to the sump giving rise to an increased pump generated pressurewhich acts through the actuator to increase the brake pressure and causedeceleration of the wheel. This increased brake pressure acts on theslip control valve urging an increase in the flow area of therestriction which will in turn decrease the brake-pressure. The wheelcan never reach the full lock-up condition inasmuch as the pump wouldstop generating pressure and the actuator would relieve the brakepressure permitting the wheel to accelerate. A low speed cutout valvecommunicates master cylinder pressure directly to the wheel brake atvehicle speeds below a'predetermined level where the pump and slipcontrol valve cannot consistently operate the brake.

The many objects, advantages and features of the present invention willbecome apparent upon consideration of the following specification andthe accompanying drawing which depicts brake control systemschematically.

Referring to the drawing, it can be seen that the brake system includesa conventional operatoractuated master cylinder which generates a mastercylinder pressure and a conventional fluid pressure actuated brake 12which imparts a braking torque to the wheel, 14 in proportion to thebrake pressure acting thereon.

A positive displacement pump 16 is drivingly connected to the wheel 14so as to provide in conduit 18 a pump generated pressure, hereinafterreferred to as A hydraulic actuator 20 serves as a hydraulic ram whichgenerates a brake pressure, hereinafter referred to as P,, in proportionto the P communicated thereto. The hydraulic actuator 20 includes apiston 22 which is sealingly movable in housing 24 and forms therewith achamber 26 to which conduit 18 communicates P,,. A piston rod 28attached to piston 22 is sealingly movable in the housing 24 and forms amovable wall of P conduit 30 which is connected to the wheel brake 12. Aspring 32 is seated on housing 24 and urges the pistons to the restposition shown in the drawing. The ratio of P,, to P, is determined bythe ratio of end areas of piston rod 28 and piston 22. Exhaust ports 34vent any leakage of hydraulic fluids.

A slip control valve 36, a conduit 48, and an orifice 38 are connectedin series between P conduit 18 and sump K7. Slip control valve 36includes a housing 44 having stepped bores 48, 50, and 52 formedtherein. A valve spool 54 is slidable in housing 44 and is sealed byseals 56, 58, and 60 with the walls or bore 48 and by seal 62 with thewall of bore 50. Valve spool 54 is necked down intermediate seals 60 and62 to provide an annular space 64 which is connected to master cylinder10 by conduit 66. It may be seen that master cylinder pressure,hereinafter referred to as P communicated to annular space 64 acts onthe differential area of spool 54 presented by the diametricaldifference between bores 48 and to provide a pressure force acting tomove spool 54 rightwardly.

An annular sleeve 68 is sealingly received in bore 52 of housing 44 andprovides a bore 70 of lesser diameter than bore 50. A piston 72 isslidable in housing 44 and sealed with the wall of bore 50 by seals 74and 76, and with the walls of bore 70 by seal 78. Piston 72 is neckeddown intermediate seals 76 and 78. providing an annular space 79. Thedifferential area of piston 72 presbe seen that spring 80 defines theposition of the valve spool 54 in housing 44 when the P pressure forceacting thereon does not exceed the combined P pressure force and forceof spring 80.

When spool 54 of slip control valve 36 is in the normal rest position asshown in the drawing, a slot 84 cut in valve spool 54 unrestrictedlycommunicates pump 16 generated flow received through conduit 18 at inletannulus 86 of housing 44 to outlet annulus 88 of housing 44 which isconnected to conduit 40. Thus, orifice 38 provides the only restrictionof .flow from P conduit 18 to the sump l7. 2

Slot 84 is of variable width so that as spool 54 is moved rightwardlyfrom the rest position, it provides a progressively decreasing flow areafrom inlet annulus 86 to outlet annulus 88. Preferably, the flow area ofthe slot 84 decreases exponentially along its portion designated A andthen provides a constant minimum flow area along the portion of itslength designated B. The significance of providing such a variable flowrestriction will be discussed hereinafter.

A low speed cutout valve 100 provides a direct fluid path between themaster cylinder 10 and the wheel brake 12 at vehicle speeds below somepredetermined level. The low speed cutout valve 100 includes a taperedplunger 102 which is slidable in housing 104 and attached to diaphragm106. A valve seat 108 formed in housing 104 connects fluid passages 110and 112 which are respectively connected to P conduit 66 and P conduit30. A hydraulically balanced piston 114 is slidable in housing 104 andis biased by spring 116 into engagement with the tapered end of plunger102. Spring '1l6 acts through piston 114 to hold plunger 102 unseatedrelative to valve seat 108 until the conduit 40 pressure communicated todiaphragm 106 by inlet port 118 exceeds the force of spring 116. Thus,as will be more fully described hereinafter, the low speed cutout valve100 provides a P,, equal to P until the conduit 40 pressure reaches apredetermined threshold during wheel acceleration and then againprovides a P equal to P during wheel deceleration. The low speed cutoutvalve 100 also includes a check valve 120 which consists of an O-ring122 seated in a groove on a plug 124 situated in the wall of housing 104between P passage 110 and P passage 112. A passageway 126 in plug 124 isclosed by an O-ring 122 when P exceeds P and is open when a P,, inexcess of P lifts the O-ring 122 from the plug 124. Thus, check valve120 prevents P,, from exceeding the level of braking being called for byP,,,

OPERATION When the vehicle is stopped, wheel 14 is not rotating and pump16 drivingly connected theretois not being driven. P in conduit [8equals zero due to its communication to sump 17 through slot 84 of slipcontrol valve 36, conduit 40, and orifice 38. Spring 32 of actuator 20holds piston 22 in its rest position shown in the drawing so that pistonrod 28 exerts no force on the hydraulic brake fluid within P conduit 30.Spring 116 holds the tapered plunger 102 of low speed cutout valve 100away from the valve seat 108 so that P,, and

P,,, both equal zero when the master cylinder is not actuated.

As the vehicle is accelerated from rest, pump 16 is driven at a speedproportional to the wheel 14 pumping fluid from sump 17 to P conduit 18.This flow in conduit 18 is communicated unrestrictedly through the fullyopen slot 84 of valve spool 54 to conduit 40 and is then communicatedrestrictedly through orifice 38 to the sump 17, giving rise to apressure P, in conduit 18. As the speed of wheel 14 and pump 16increases with the vehicle speed, P, increases to a point where thespeed reference pressure in conduit 40 acting on diaphragm 106 of lowspeed cutout valve 100 exceeds the force of spring 116 thus closing thelow speed cutout valve 100. Spring 32 of the actuator 20, orifice 38,and pump 16 are sized relative one another to prevent movement ofactuator piston 22 by the P pressure force throughout the range ofoperating speeds of the vehicle.

During master cylinder actuation at wheel speeds below the threshold oflow speed cutout valve 100, the master cylinder generated pressure P iscommunicated through the low speed cutout valve 100 directly to wheelbrake 12. At wheel speeds above the threshold of low speed cutout valve100, pump 16 provides hydraulic power for actuation of the brake 12through actuator 20 with the slip control valve 36 functioning tocontrol the magnitude of P acting on the actuator 20.

The position of valve spool 54 within housing 44 and consequently theflow area presented by slot 84 is determined by the difference betweenP,, and P and the force of spring 80. P,,, in annular space 64 urgesvalve spool 54 rightwardly to decrease the flow area giving rise to anincreased P which will in turn increase P,, through the action ofactuator 20. P,, in annular space 79 acts on piston 72 urging theabutting spool 54 leftwardly to increase the flow area presented by slot84 and thus reduce P,, and P,,

Assuming that the vehicle is travelling at a given speed above thethreshold of low speed cutout valve 100, and the master cylinder 10 isfully actuated, P, will move valve spool 54 rightwardly so that the flowarea of slot 84 substantially restricts the exhausting of P,, to orifice38. The resultant P generated by pump 16 acts through modulator 20 toprovide a P of such magnitude that the wheel 14 may lock up. This P actsleftwardly on spool 54 against the P pressure force and urges spool 54to increase the flow area presented by slot 84. Before the wheel locksup, it must slow down and pump 16 is slowed in synchronism. The P,, willbe decreased with wheel speed and actuator 20 will accordingly decreaseP In this manner the speed of wheel 14 is made to decrease insynchronism with the vehicle without locking up, but at a wheel slipratio controlled by the operator.

When the wheel 14 has slowed to the predetermined low speed, the speedreference pressure in conduit 40 will have dropped below the thresholdpressure of low speed cutout valve 100 so that spring 116 unseatstapered plunger 102. At this point, P from master cylinder 10 iscommunicated through the low speed cutout valve directly to P,, conduit30 and the wheel brake 12. Thus braking effort is maintained even thoughthe wheel 14 and the pump 16 are completely stopped.

The exponentially decreasing flow area portion A of slot 84 and theminimum area portion B of the slot 84 provide a decreasing sensitivityin the slip control valve 36. When the differential between mastercylinder pressure P, and brake pressure P,, is increased, the wheel slipalso increases. The flow area presented by slot 84 decreases at anexponentially decreasing rate as the spool 54 moved rightwardly underthe influence of the increasing differential between P and P,, so thatthe flow area becomes less and less sensitive to the differential as itincreases. In this manner the operator may control the level of wheelslip but can never cause the wheel to lock-up altogether.

The variable sensitivity feature of slip control valve 36 mayalternately be provided by a slot 84 having a flow area which decreaseslinearly followed by a portion having a constant minimum flow area. Inthis case, the spring would be of variable rate so that the flow areawould decrease at a decreasing rate when the spool 54 is movedrightwardly.

We claim:

1. In a vehicle having a wheel brake system including a fluid pressureoperated wheel brake and an operator actuated master pressure generatingdevice the combination of:

a positive displacement pump connected to the wheel for synchronousoperation therewith;

actuator means providing a brake pressure to the wheel brake inproportion to the pump generated pressure;

orifice means exhausting the pump generated pres- 7 sure so thatthewheel brake is'not normally actuated throughout the range of wheelspeed;

slip control valve means interposed fluidly intermedition of thedifference between master pressure and brake pressure, the housing andvalve spool being ported relative to one another to provide uponmovement of the spool from the normal position a flow area decreasingexponentially to a constant minimum flow area so that the change in flowarea becomes progressively less responsive to the increasingdifferential between master pressure and brake pressure;

and normally open valve means communicating master pressure to the wheelbrake and including means responsive to a predetermined fluid pressureexisting fluidly intermediate the slip control valve and the orificemeans to close the normally open

1. In a vehicle having a wheel brake system including a fluid pressureoperated wheel brake and an operator actuated master pressure generatingdevice the combination of: a positive displacement pump connected to thewheel for synchronous operation therewith; actuator means providing abrake pressure to the wheel brake in proportion to the pump generatedpressure; orifice means exhausting the pump generated pressure so thatthe wheel brake is not normally actuated throughout the range of wheelspeed; slip control valve means interposed fluidly intermediate the pumpand the orifice means and including a housing, a valve spool slidablefrom a normal position to progressively restrict fluid flow from thepump to the orifice means, linear rate spring means urging the valvespool to the normal position, means introducing master pressure into thehousing To urge the spool from the normal position, and meansintroducing brake pressure into the housing to urge the spool toward thenormal position to vary the magnitude of the flow restriction as afunction of the difference between master pressure and brake pressure,the housing and valve spool being ported relative to one another toprovide upon movement of the spool from the normal position a flow areadecreasing exponentially to a constant minimum flow area so that thechange in flow area becomes progressively less responsive to theincreasing differential between master pressure and brake pressure; andnormally open valve means communicating master pressure to the wheelbrake and including means responsive to a predetermined fluid pressureexisting fluidly intermediate the slip control valve and the orificemeans to close the normally open valve means.